It uses WiFi connections between the DS and Wii. Basically its a wireless connection in which you use the Wii via the DS and vice versa to play certain games and browse the internet on both the Wii or DS. I don't know any more info apart from what I stated.

Nintendo DS connectivity
The Wii console supports wireless connectivity with the Nintendo DS. Shigeru Miyamoto said Nintendo was still working out when features using this connectivity would be available, but that it would be soon after the launch of the system, due to the popularity of the Nintendo DS. At Nintendo's corporate policy meeting in June 2006, Satoru Iwata explained that the DS uses its wireless connectivity to communicate with Wii and that no further accessories will be needed.
The connectivity will allow the player to use functions like the Nintendo DS' microphone and touchscreen as inputs for Wii games. The first example Nintendo has given of a game using Nintendo DS-Wii connectivity is that of Pokémon Battle Revolution. Players with either Pokémon Diamond or Pearl will be able to play battles using their Diamond or Pearl Pokémon on Wii with the Nintendo DS as a controller.
It has also been confirmed that the Nintendo DS will be able to play game demos downloaded from the Wii console which they would receive from Nintendo, similar to a DS Download Station. The console will also be able to expand Nintendo DS games

thanks steww good info im really looking forward to seeing an aresnal of games that will benefit form this compatitbility. This is one of the biggest reasons i want a DS now, other then a few games I really wanna play for it. And of course Phantom Hourglass coming out in 07.